The man who gave the Leave campaign the biggest donation in British political history has said he would not vote for Brexit today because the EU referendum had “unleashed demons”.

Arron Banks insisted his £8 million worth of support for the Leave.EU campaign he founded did not come from Russia and was completely legal, after a probe into the funds was opened by the National Crime Agency (NCA) on the recommendation of the UK elections watchdog.

Leave campaigner Arron Banks appeared on the Andrew Marr show on Sunday morning. Picture: PA

In a combative appearance on the BBC’s Andrew Marr Show, Mr Banks insisted there was “no Russian money and no interference”, but contradicted an earlier statement about which of the companies in his insurance business was the source of the funds.

Mr Banks claimed the investigation triggered by the Electoral Commission was part of a campaign to “smear” him and other leading Brexit campaigners in a bid to stop the UK leaving the EU.

Asked if he would back the Leave side in a rerun of the 2016 referendum, Mr Banks said: “The corruption I have seen in British politics, the sewer that exists and the disgraceful behaviour of the Government over what they are doing with Brexit and how they are selling out, means that if I had my time again I think we would have been better to probably remain and not unleash these demons.”

The NCA probe was launched into “suspected criminal offences” after the elections watchdog said it had reasonable grounds to suspect that Mr Banks was not the true source of the cash.

It also said that the loans involved a company – Rock Holdings, based on the Isle of Man, which was impermissible under finance rules that say all campaign funds during an election must come from within the UK.

But Mr Banks told Marr that it came from a UK-based insurance business, saying: “There was no Russian money and no interference of any type. I want to be absolutely clear about that.

“The money came from Rock Services, which was a UK limited company. It was generated out of insurance business written in the UK.

“Contrary to some of the press reports in the Financial Times and other Remain-leaning publications, we insure nearly half a million customers a year – the size of Manchester. We turn over £250 million of premiums. It’s a sizeable business.”

The explanation differs from one Mr Banks gave the Commons Culture, Media and Sport (DCMS) Committee in June, in which he claimed Rock Services was “just a service company”.

He told MPs: “The actual loan came from one of my companies that was delivered in.”

The Electoral Commission’s review of referendum finances focused on £2 million reported to have been loaned to Better for the Country by Mr Banks and his insurance companies and a £6 million donation which he made alone.

Mr Banks lashed out again at the Electoral Commission, accusing it of being full of his political opponents, as well as elements of the media who opposed Brexit.

He said Rock Services had “all sorts of revenues”, but did not detail them.

“I know it’s complex for journalists to understand but we know what this is about – it’s about undermining Article 50 and the Brexit result,” he added.

“It’s a group of vicious MPs who have grouped together with the Guardian and the FT.”

Mr Banks also attacked Damian Collins, chairman of the DCMS Committee. Asked why he had written to Mr Collins’s constituents in Folkestone and Hythe, calling him a “snake in the grass”, Mr Banks replied: “Well, he is.”

The commission also referred Leave.EU, its chief executive, Elizabeth Bilney, and the organisation that ran it, Better for the Country, to the NCA last week after carrying out a review.

Bob Posner, the commission’s director of political finance, said: “We have reasonable grounds to suspect money given to Better for the Country came from impermissible sources and that Mr Banks and Ms Bilney, the responsible person for Leave.EU, knowingly concealed the true circumstances under which this money was provided.

“This is significant because at least £2.9 million of this money was used to fund referendum spending and donations during the regulated period of the EU referendum.”

Ms Bilney also denies any wrongdoing.

Liberal Democrat Brexit spokesman Tom Brake said Mr Banks’ appearance on the Andrew Marr Show had been “remarkable”.

“His evasive approach to questions highlights the culture of dodgy deals in the Brexiteers’ campaign. The campaign to leave the EU was jam-packed full of lies, deceit and allegations of much worse.

“Even Banks has admitted that Brexit was a mistake, stating that the shambles we are in now shows we would be better off staying in the EU.

“The Conservatives must give the people the final say on Brexit and a chance to remain in the EU.”